1. Field of the Invention
This invention relates to composite moulding materials or pre-forms of resin and reinforcement fiber in which the resin is only partially impregnated into the fiber.
2. Description of Related Art
Such pre-forms are normally supplied in a continuous roll but may also be provided as pre-cut sheets. In either case the material may be laid up in multiple layers on a shaped moulding surface, alone or in combination with further resin, and then processed (e.g., by application of heat and pressure) to form strong, lightweight structures, e.g., for use in forming panels or body sections or components in aerospace, automotive, wind energy or other contexts.
The fibrous reinforcing material may be partially or substantially fully impregnated with the resin in the pre-form, resulting in a so-called semi-preg or pre-preg. The fibrous material advantageously may comprise tows with fibers which are unidirectional, i.e., which extend predominantly in a common direction, since this facilitates manufacture of strong laminates of high mechanical integrity suited to the production of relatively long, large and thick structures such as wind energy blades.
Semi-pregs in which the fibrous material is only partially resin-impregnated so as to leave a ‘dry’ outer surface, i.e., a surface having free fibers not impregnated by resin, can advantageously facilitate ready release of trapped air during processing which minimizes void content and consequent mechanical weakening in the finished product. However, there is the problem that a unidirectional alignment of the exposed dry fibers on the outer surface tends to become disrupted during handling, cutting and laying-up with consequent reduction in mechanical integrity of the finished product. Moreover, contaminant fibrous debris may be produced.
Disruption of fiber alignment can be avoided with conventional pre-pregs due to the full resin impregnation of the fibrous material, but with these materials, problems of handling and void content can arise particularly when using heavy fiber bundles.
WO 00/27632 provides an example disclosure of a semi-preg which has exposed dry fibers and which would therefore exhibit the above mentioned fiber-disruption problem.
EP 0583090A describes sandwiching of fibrous material between resin films. The fibrous material is impregnated by the resin matrix which would prevent surface disruption of fibers. However, this technique, particularly if used with substantial unidirectional fiber tows, would tend to give only partial impregnation thereby leading to air entrapment in the dry fibers in the center of the pre-form causing voids and consequent poor mechanical strength in the finished product.
With a view to improving mechanical integrity, it is known to cross-stitch unidirectional fiber tows. This is described in WO98/34979 in the context of a semi-preg. However, loose or broken filaments from the stitching process can cause performance deterioration in the laminate and also does not necessarily solve the handling problems associated with unidirectional tows.
Warp fiber bundles can be held together in pre-forms with thermoplastic stabilizing yarns. However, if the melting point of the thermoplastic is higher than the processing temperature of the pre-form resin matrix, the yarns can give rise to fiber distortion, and hence poor mechanical properties, where the weft yarns are crimped over the warp. If the thermoplastic has a lower melting point than the resin matrix processing temperature, the thermoplastic melts and flows into the resin matrix producing voids in the finished product.
U.S. Pat. No. 6,096,669 describes the formation of a pre-form obtained by sandwiching unidirectional fiber tows between resin films which are in the form of openwork diamond-pattern grids. The grids act to hold the fibers in position but are relatively insubstantial and a resin matrix is introduced externally by penetration through the grids using a conventional so-called resin transfer moulding process. Such processes are used for production of relatively small, short length structures and are not appropriate for larger, longer length structures, where adequate resin flow along the fiber pre-form cannot be ensured.
U.S. Pat. No. 4,213,930 describes the use of a gravure roll to apply small amounts of resin to the surface of a pre-preg to improve surface tack on a fully impregnated pre-form.